1. Field of the Invention
The present invention relates to a display device, and more particularly to a liquid crystal display device which forms pixels using thin film transistors (TFTs) as switching elements in a display region and arranges a driver circuit formed of a TFT whose channel portion is made of poly-Si on the periphery of the display region.
2. Description of the Related Art
In a liquid crystal display device, a TFT substrate on which pixel electrodes, thin film transistors (TFTs) and the like are formed in a matrix array and a color filter substrate on which color filters and the like are formed at positions corresponding to the pixel electrodes are arranged to face each other in an opposed manner, and liquid crystal is sandwiched between the TFT substrate and the color filter substrate. Optical transmissivity of liquid crystal molecules is controlled for every pixel so as to form an image.
On the TFT substrate, data lines which extend in the longitudinal direction and are arranged parallel to each other in the lateral direction and scanning lines which extend in the lateral direction and are arranged parallel to each other in the longitudinal direction are formed, and the pixel is formed in a region surrounded by the data lines and the scanning lines. The pixel is mainly constituted of a pixel electrode and the TFT which constitutes a switching element. A display region is formed by arranging a large number of pixels having such constitution in a matrix array.
Outside the display region, a scanning line driver circuit which drives the scanning lines and a data line driver circuit which drives the data lines are arranged. Conventionally, the scanning line driver circuit and the data line driver circuit are formed by externally mounting an IC driver. This IC driver may be connected to the TFT substrate using a tape carrier method or the like or may be directly mounted on the TFT substrate using a chip-on method.
On the other hand, to satisfy the demand for the miniaturization of the whole display device while ensuring the display region or the like, a technique which forms a driver circuit on the periphery of the display region using a TFT has been developed. In such a display device, the TFT formed in the display region uses amorphous silicon (a-Si) for forming a channel portion thereof, and the TFT formed in the driver circuit part uses polysilicon (poly-Si) for forming a channel portion thereof. That is, a-Si which exhibits a small leak current is used for forming the channel portion of the TFT in the display region, while poly-Si which exhibits large mobility of electrons is used for forming the channel portion of the TFT in the driver circuit part.
In general, the TFT which uses a-Si for forming the channel portion adopts the bottom gate structure, while the TFT which uses poly-Si for forming the channel portion adopts the top gate structure. Accordingly, the TFTs which differ in structure are formed on one substrate and hence, the manufacturing process becomes complicated.
JP-A-5-55570 (patent document 1) discloses a display device which is, to prevent a manufacturing process from becoming complicated, configured such that a TFT which uses poly-Si for forming a channel portion thereof also adopts the bottom gate structure. In this constitution, a poly-Si layer which constitutes a channel is firstly formed on a gate insulation film which is formed on a gate electrode and, thereafter, an a-Si layer is formed on the poly-Si layer. A contact layer which is constituted of an n+ layer is formed on the a-Si layer, and a source/drain electrode (SD electrode) is formed on the contact layer. By allowing the TFT which uses poly-Si for forming the channel portion thereof to adopt such constitution, the number of common processes which are shared by a TFT which uses a-Si for forming a channel portion thereof and the TFT which uses poly-Si for forming the channel portion thereof is increased and hence, the entire process is simplified.